You know the type, those you can put a button for in your car, allowing you to open the garage door without getting out of your car when you get home. I worry that these might not be fully secure if you keep anything in your garage that's worth stealing. Surely a thief in the know could create a device that sends out the same signal, allowing him to enter your garage? What are the safeguards in these systems?

It depends on the type of signal they use. If they use a 'fixed code', they can be hacked in seconds by a crafty thief with brains and specialist equipment. However, if they use a 'rolling code', they're generally safe from this and other attacks.